Introduction to Polymer Chemistry
Classifications, Preparation of Commercial Polymers & Solved Board PYQs
1. Introduction and Classification
Polymers are high molecular mass macromolecules ($10^3 - 10^7$ u) formed by the repeated linking of small fundamental units called monomers. The process of formation of polymers from respective monomers is called polymerization.
A. Classification based on Source
- Natural Polymers: Found in plants and animals. Examples: Cellulose, starch, proteins, natural rubber.
- Synthetic Polymers: Man-made polymers synthesized in the laboratory. Examples: Polythene, Nylon, Teflon, Bakelite.
- Semisynthetic Polymers: Derived from naturally occurring polymers by chemical modifications to improve their properties. Examples: Cellulose acetate (Rayon), Cellulose nitrate, Vulcanized rubber.
B. Classification based on Structure
- Linear Polymers: Monomers are linked to form long, straight chains. They have high density, high melting point, and high tensile strength due to close packing. Examples: High Density Polythene (HDPE), PVC.
- Branched Chain Polymers: Contain linear chains with some branches. Due to irregular packing, they have lower density and lower melting points. Example: Low Density Polythene (LDPE).
- Cross-linked (Network) Polymers: Formed from bi-functional or tri-functional monomers, creating a strong 3D network of covalent bonds. They are hard, rigid, and brittle. Examples: Bakelite, Melamine.
2. Classification based on Molecular Forces
Mechanical properties of polymers depend on intermolecular forces (van der Waals forces, hydrogen bonding). They are classified into four groups:
| Category | Characteristics & Intermolecular Forces | Examples |
|---|---|---|
| Elastomers | Rubber-like elastic properties. Weakest intermolecular forces permit the polymer to be stretched. A few cross-links act like springs. | Natural rubber, Buna-S, Buna-N, Neoprene. |
| Fibres | Thread-forming solids with high tensile strength. Strongest intermolecular forces (like Hydrogen bonding or dipole-dipole interactions). | Nylon-6,6, Terylene (Dacron), Silk. |
| Thermoplastic | Soften on heating and harden on cooling (can be remolded). Forces are intermediate between elastomers and fibres. Usually linear or slightly branched. | Polythene, Polystyrene, PVC. |
| Thermosetting | On heating, they undergo extensive cross-linking in moulds and become infusible. Cannot be remolded. They are hard and infusible. | Bakelite, Urea-formaldehyde resin. |
3. Preparation of Important Addition Polymers
Formed by the repeated addition of monomer molecules possessing double or triple bonds without the elimination of any byproduct.
A. Polythene (Polyethylene)
Monomer: Ethene ($CH_2=CH_2$).
- Low Density Polythene (LDPE): Obtained by heating ethene at 350-570 K under high pressure (1000-2000 atm) with traces of oxygen or peroxide initiator. It has a highly branched structure. It is chemically inert, tough but flexible. Used in squeeze bottles, flexible pipes, and carry bags.
- High Density Polythene (HDPE): Obtained by heating ethene at 333-343 K under low pressure (6-7 atm) in the presence of Ziegler-Natta catalyst ($TiCl_4 + (C_2H_5)_3Al$). Consists of linear chains. It is much stiffer, harder, and has higher tensile strength than LDPE. Used in buckets, dustbins, and pipes.
B. Teflon (Polytetrafluoroethylene - PTFE)
Monomer: Tetrafluoroethene ($CF_2=CF_2$).
$n CF_2=CF_2 \xrightarrow{\text{Catalyst, High P}} [-CF_2-CF_2-]_n$
Properties/Uses: Chemically inert, resistant to attack by corrosive reagents. Used in making non-stick cookware, oil seals, and gaskets.
C. Polyacrylonitrile (PAN) or Orlon
Monomer: Acrylonitrile ($CH_2=CH-CN$).
$n CH_2=CH-CN \xrightarrow{\text{Peroxide}} [-CH_2-CH(CN)-]_n$
Uses: Used as a substitute for wool in making commercial fibres like Orlon or Acrilan (blankets, sweaters).
4. Condensation Polymers (Step-Growth Polymers)
Formed by repeated condensation reaction between two different bi-functional or tri-functional monomers, with the elimination of simple molecules like $H_2O, NH_3$, or $HCl$.
A. Nylon-6,6 (A Polyamide)
Monomers: Hexamethylenediamine and Adipic acid.
$n H_2N-(CH_2)_6-NH_2 + n HOOC-(CH_2)_4-COOH \xrightarrow{553 \text{ K, High P}}$
$[-NH-(CH_2)_6-NH-CO-(CH_2)_4-CO-]_n + 2n H_2O$
Properties/Uses: It has high tensile strength due to strong intermolecular hydrogen bonding. Used in making sheets, bristles for brushes, and in the textile industry.
B. Nylon-6
Monomer: Caprolactam.
Prepared by heating caprolactam with water at high temperature to open the ring, followed by polymerization.
Uses: Manufacture of tyre cords, fabrics, and ropes.
C. Terylene / Dacron (A Polyester)
Monomers: Ethylene glycol and Terephthalic acid.
$n HO-CH_2-CH_2-OH + n HOOC-C_6H_4-COOH \xrightarrow{420-460 \text{ K, Catalyst}}$
$[-O-CH_2-CH_2-O-CO-C_6H_4-CO-]_n + 2n H_2O$
Uses: Crease-resistant. Used in blending with cotton (terycot) and wool (terywool), and for making magnetic recording tapes.
D. Bakelite (Phenol-Formaldehyde Polymer)
Monomers: Phenol and Formaldehyde ($HCHO$).
Initially forms a linear polymer called Novolac (used in paints). On heating with hexamethylenetetramine, Novolac undergoes cross-linking to form an infusible solid mass called Bakelite. It is a thermosetting polymer.
Uses: Making combs, electrical switches, handles of utensils, and computer discs.
5. Natural Rubber and Vulcanization
Natural Rubber is a linear polymer of Isoprene (2-methyl-1,3-butadiene). Its chemical name is cis-1,4-polyisoprene. It is a natural elastomer.
Vulcanization of Rubber
Natural rubber is soft, sticky, and has low tensile strength. To improve its physical properties, it is heated with sulfur (3-5%) at 373 K to 415 K. This process was invented by Charles Goodyear.
Chemistry of Vulcanization: Sulfur forms rigid 3D cross-links at the reactive sites of double bonds. This restricts the movement of polymer chains over each other, making the rubber stiffer, highly elastic, and resistant to wear and tear.
6. Biodegradable Polymers
A major drawback of synthetic polymers is that they are non-biodegradable, causing severe environmental pollution. Biodegradable polymers degrade in the environment by bacterial action.
A. PHBV (Poly $\beta$-hydroxybutyrate - co - $\beta$-hydroxyvalerate)
Monomers: 3-Hydroxybutanoic acid and 3-Hydroxypentanoic acid.
It is an aliphatic polyester. Used in specialty packaging, orthopedic devices, and controlled release of drugs.
B. Nylon-2-nylon-6
Monomers: Glycine ($H_2N-CH_2-COOH$) and Aminocaproic acid ($H_2N-(CH_2)_5-COOH$).
It is a biodegradable alternating polyamide.
7. Solved Textbook Reasoning Questions
Question 1: Thermoplastic vs Thermosetting
Explain the difference between thermoplastic and thermosetting polymers on the basis of heating effect.
Answer:
Thermoplastic Polymers: These are linear or slightly branched long-chain molecules. On heating, they become soft and melt, and on cooling, they harden. This process is reversible, so they can be reshaped and recycled repeatedly (e.g., Polythene, PVC).
Thermosetting Polymers: These are heavily branched and cross-linked molecules. Upon heating in a mould, they undergo extensive cross-linking to form a hard, infusible, and insoluble 3D network. This chemical change is permanent. They cannot be softened, remolded, or reused (e.g., Bakelite).
Question 2: Ziegler-Natta Catalyst
What is the composition of the Ziegler-Natta catalyst and where is it used?
Answer:
The Ziegler-Natta catalyst is a mixture of Titanium tetrachloride ($TiCl_4$) and Triethylaluminium ($(C_2H_5)_3Al$). It is used in the preparation of High-Density Polythene (HDPE) from ethene at low pressures (6-7 atm) and low temperatures (333-343 K).
8. Board PYQs with Complete Answers
Verified previous year questions from the Maharashtra State Board HSC Chemistry exams.
1 Mark Questions (VSA)
Q1. Write the name of the monomer of natural rubber. (March 2013, Oct 2017)
Answer: Isoprene (2-methyl-1,3-butadiene).
Q2. Name the polymer used for making non-stick cooking utensils. (March 2016, Oct 2021)
Answer: Teflon (Polytetrafluoroethylene).
Q3. Give one example of a biodegradable polymer. (March 2018, March 2022)
Answer: PHBV (or Nylon-2-nylon-6).
2 Mark Questions (SA-I)
Q4. What is vulcanization of rubber? Why is it carried out? (March 2014, March 2020)
The process of heating natural rubber with sulfur (3-5%) at 373-415 K to introduce sulfur cross-links between the polymer chains is called vulcanization.
It is carried out to improve the physical properties of natural rubber, making it harder, stronger, more elastic, and less sticky.
Q5. Write the names and structures of the monomers used in the preparation of Nylon-6,6. (Oct 2015, March 2019)
- Hexamethylenediamine: $H_2N-(CH_2)_6-NH_2$
- Adipic acid: $HOOC-(CH_2)_4-COOH$
3/4 Mark Questions (LA)
Q6. Distinguish between addition polymers and condensation polymers. Give one example of each. (Oct 2014, March 2018, March 2023)
| Feature | Addition Polymers | Condensation Polymers |
|---|---|---|
| Formation | Formed by repeated addition of monomer molecules possessing double/triple bonds. | Formed by repeated condensation reaction between two bi-functional monomers. |
| Byproducts | No elimination of byproducts occurs. | Elimination of small molecules like $H_2O, NH_3, HCl$ occurs. |
| Monomers | Usually involve one type of unsaturated monomer. | Involve two different bi-functional monomers. |
| Example | Polythene, Teflon, PVC | Nylon-6,6, Terylene, Bakelite |
Q7. What are elastomers? Give two examples. Write the reaction for the preparation of Terylene. (March 2015, Oct 2020)
Elastomers:
Polymers that have rubber-like elastic properties. In these polymers, the chains are held together by the weakest intermolecular forces, allowing them to be stretched under stress. A few cross-links are present between the chains, helping them retract to their original position when the stress is removed.
Examples: Natural rubber, Buna-S, Neoprene.
Preparation of Terylene (Dacron):
Prepared by the condensation polymerization of ethylene glycol and terephthalic acid at 420-460 K.
$n HO-CH_2-CH_2-OH + n HOOC-C_6H_4-COOH \xrightarrow{\Delta}$
$[-O-CH_2-CH_2-O-CO-C_6H_4-CO-]_n + 2n H_2O$
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